The present invention relates to a hydraulic brake system for automotive vehicles including an actuating unit comprising a master brake cylinder and a preceding vacuum brake power booster operable by a brake pedal, wheel brakes which are connected to the pressure chambers of the master brake cylinder by the intermediary of an ABS hydraulic unit, wheel brakes which are connected to the pressure chambers of the master brake cylinder by the intermediary of an ABS hydraulic unit, a brake light switch which interacts with the brake pedal, an electric switching device which permits identification of the driver""s wish to the end of reducing the braking effect, a sensor device for sensing the brake pedal actuating speed, and an electronic control unit which is furnished with the electric signals of the brake light switch, the electric switching device and the sensor device, and the output signals of which permit activating a pneumatic valve, which controls the build-up of a pneumatic differential pressure in the housing of the vacuum brake power booster, irrespective of the operation by the driver, to the effect of full braking.
A brake system of this type is disclosed in German patent No. 42 08 496, for example. The means of sensing the actuation speed in the state of the art brake system is associated with the brake pedal and, preferably, is a pedal position sensor configured as an angle sensor. The above mentioned switching device is arranged in the area of articulation of, the actuating rod to the brake pedal and is configured as a relative motion sensor. The arrangement of the two sensor devices in the driver""s leg room causes considerable cost of assembly which is especially due to the fact that a cable tree must be positioned in the mentioned area. Further, there is the imminent risk that the function of the prior art sensor devices is impaired e.g. by contaminants or moisture in the driver""s leg room.
Therefore, an object of the present invention is to improve a hydraulic brake system of the previously mentioned type so that assembly costs, in particular related to installation of the brake system, are considerably reduced and reliability of its operation is additionally increased.
According to the present invention, this object is achieved because the sensor device is configured as a travel sensor of analog operation which is operable by the axial movement of the movable wall of the vacuum brake power booster, the output signal of the travel sensor being directly proportional to the actuating travel of the movable wall and being subjected to a time differentiation processing operation in the electronic control unit. It is particularly appropriate when the travel sensor is a linear potentiometer.
In an embodiment of the present invention related to a particularly compact size, the pneumatic valve is the control valve of the vacuum brake power booster which is operable by the brake pedal and is additionally operable by electromechanical means adapted to be driven by the electronic control unit.
In a preferred aspect of the subject matter of the present invention, the electromechanical means is a solenoid and a sealing seat of the control valve interacting with the solenoid. The solenoid is a component part of a valve piston which is in a force-transmitting connection with the brake pedal. It is achieved by these measures that the electric lines leading to the travel sensor may be integrated in the cable which extends to the solenoid. This ensures that space is optimally used.
In another preferred aspect of the present invention which permits low-cost manufacture, the electric switching device is arranged in a control housing accommodating the control valve. Preferably, the switching device includes a microswitch on the valve piston and an actuating element which is slidable in the control housing and bears against a stop on the booster housing in the inactive position of the vacuum brake power booster.
Identification of the position of the brake pedal is important for the proper functioning of the independent-force-assisted brake system of the present invention because the release threshold for the independent actuation varies with an increasing brake pedal travel. The actual position of the brake pedal, however, is a function of the vacuum prevailing in the housing of the vacuum brake power booster, which causes advance movement of the movable wall generating the boosting force, and of the degree of venting of the brake system. The travel sensor must be calibrated, i.e. its zero point or initial point must be defined, with respect to the importance of the absolute pedal travel. However, one-time calibration during the assembly of the device is not sufficient, because shifts caused by wear may occur in the service life of the system. Also, a variation of the position of the travel sensor in its attachment is not identified in a one-time calibration of the travel sensor. Therefore, the electronic control unit has a means in another preferred aspect of the present invention which permits a continuous calibration of the travel sensor. The sensed calibration value (zero point of the travel sensor) is stored preferably in dependence on the switch condition of the brake light switch and the electric switching device, or only if both the brake light switch and the electric switching device are in their non-actuated condition.
The reliability in operation of the brake system of the present invention is further increased by a means of the electronic control unit which permits identifying the calibration performed.
Further details, features and advantages of the present invention can be seen in the following description of an embodiment, making reference to the accompanying drawings in which individual parts corresponding to each other have been assigned like reference numerals.